Nitrogen oxides are present in exhaust gases from stationary sources such as power plants, industrial processes, simple cycle and combined cycle gas turbines, gas engines, and diesel engines. Emissions from these sources are subject to regulations in both the United States and Europe. The laws which regulate emission standards have become stricter in recent years and will continue to become stricter in the future. Many countries require 90% NOx removal. Modern gas turbines generally emit about 25 ppm NOx, thus the NOx level needs to be reduced to about 2.5 ppm in order to comply with governmental regulations.
Diesel engines normally operate at air to fuel ratios above stoichiometric. Emissions of nitrogen oxides and particulates from diesel-fueled vehicles may be significant. Emissions from diesel vehicles are also subject to regulations in both the United States and Europe.
One method of removing NOx in exhaust gas is to contact the exhaust stream with a reductant such as ammonia in the presence of a catalyst at elevated temperature. The catalyzed reaction of the reductant with NOx is referred to as selective catalytic reduction (SCR). Urea, ammonium hydroxide, ammonium formate, and other nitrogen-containing chemicals can also be used as a source of ammonia.
The temperature of the exhaust gas is critical in determining the type of catalyst that can be used because certain catalysts function (or function best) in certain temperature ranges. Exhaust gases from diesel engines are typically rather low, about 200° C., while the exhaust gases from gas turbines and gas engines are typically in the range of about 300° C. to about 700° C. Thus, catalysts used with diesel applications are not likely to work effectively when used with gas turbines or gas engines and vice versa.
Traditional ammonia SCR catalysts are based on vanadia/titania. Imanari, et al. (U.S. Pat. No. 4,833,113), for example, describe an SCR catalyst comprising an oxide of titanium, an oxide of tungsten, and an oxide of vanadium. Vanadia/titania ammonia SCR catalysts normally operate at a temperature of about 250-370° C. Exhaust gas from light duty diesel vehicles is normally at a temperature of about 200° C. or less. Vanadia/titania SCR catalysts do not have significant activity at temperatures as low as 200° C. or at high temperatures.
Byrne (U.S. Pat. No. 4,961,917, assigned to Engelhard Corporation) discloses a method of passing ammonia, nitrogen oxides, and oxygen over iron or copper-promoted zeolite catalysts to selectively catalyze the reduction of the nitrogen oxides. The fresh copper-promoted catalyst has good activity. However, the copper catalyst deactivates significantly when aged. Although the iron catalyst is far more stable than the copper catalyst, it has maximum activity at about 350-500° C., significantly higher than the 200° C. temperatures that occur in many diesel exhaust streams. In addition, the copper and iron catalysts have low activity at temperatures greater than 500° C., which often occur with gas engines and gas turbines, and are not very efficient in reducing NOx.
Ito et al. (U.S. Pat. No. 5,900,222) describes a process for treating NOx containing gases at temperatures in the range of 300-560° C. using a cerium containing zeolite catalyst and a reducing agent. The catalyst described in Ito was designed to be used primarily with diesel exhaust gases. Diesel engines generally emit much higher levels of NOx than do gas turbines and gas engines. Because the rate of NOx conversion is generally a function of how much NOx is present to be reduced, catalysts effective for reducing high amounts of NOx may not be effective for reducing low amounts of NOx and vice versa.
Ichiki et al. (U.S. Patent Application Publication No. 2005/0159304) teach denitration catalysts for use at high temperatures. Multiple catalysts are described. One contains a composite oxide composed of titanium oxide and at least one of tungsten oxide, molybdenum oxide and boron oxide. Another catalyst contains zirconium oxide and SO3 or SO42−. The final catalyst described contains at least one of tungsten oxide, molybdenum oxide and boron oxide supported on a carrier comprising zirconium oxide and SO3 or SO42−. No long-term data is provided. Some of the drawbacks seen in some of the catalysts described include the use of sulfuric acid in preparing the catalyst and the possibility that such catalysts will not be active long-term.
There is a need for ammonia SCR catalysts that are stable toward aging and that are active at high temperatures and are effective in further reducing low levels of inlet NOx.